1. Field of the Invention
The present invention relates to a variable-cycle engine which selectively operates in a two-cycle mode and a four-cycle mode depending on the rotational speed of the engine and the load on the engine.
2. Description of the Prior Art
Ordinary reciprocating engines are roughly grouped into two-cycle engines in which the intake, compression, power, and exhaust strokes are performed while the pistons reciprocate one stroke, i.e., the crankshaft makes one revolution and four-cycle engines in which the above four strokes are carried out while the pistons reciprocate two strokes, i.e., the crankshaft makes two revolutions.
The two-cycle engines are generally of the uniform-flow type in which intake ports are positioned in a lower portion of a cylinder sleeve, and intake air is introduced and exhaust gases are discharged simultaneously by air supplied under pressure from the intake ports when the piston is lowered. Since the explosion occurs each time the crankshaft makes one revolution, the rotational speed of the output shaft suffers less fluctuations, and the engine can produce a high-torque output.
In the four-cycle engines, intake air is drawn and exhaust gases are discharged in respective independent strokes. Therefore, the intake air and the exhaust gases are well exchanged in a high engine speed range. Accordingly, the four-cycle engine has a low fuel consumption rate when the engine speed is high.
There has been an attempt to operate an engine selectively in a two-cycle mode and a four-cycle mode so that the engine can operate with different two- and four-cycle characteristics. Since the intake ports used in the two-cycle mode are positioned in the lower portion of the cylinder sleeve, the engine is required to have a special mechanism for preventing the interior and exterior of the cylinder from communicating with each other through the intake ports when the piston is lowered during operation of the engine in the four-cycle mode.
If the opening area of the intake ports is increased for increasing the intake efficiency thereof during operation of the engine in the two-cycle mode, then the expansion stroke is shortened to reduce the engine output power, and the intake air tends to flow back when the engine rotates at high speed.
The applicant has proposed a variable-cycle engine which has a sleeve valve disposed around a cylinder sleeve for opening and closing intake ports defined in the cylinder sleeve, the sleeve valve being actuatable by an electro-magnetic solenoid through a link to open and close the intake ports as desired (see Japanese Patent Application No. 1(1989)-112507).
The proposed mechanism is however relatively complex. The sleeve valve cannot be moved with a quick response because of the inertia of the sleeve valve itself, and gaps or clearances between the movable parts and also between the movable parts and fixed parts supporting the movable parts.